The present invention relates to a system for controlling a lock-up clutch.
A system for controlling a lock-up clutch of the prior art is disclosed, for example, in U.S. Pat. No. 4,730,521 issued to Hayasaki et al. or the corresponding Japanese Provisional Patent Publication No. 62-62047. This system comprises a lock-up control valve which controls an operation of a lock-up clutch of a torque converter. The lock-up control valve is controlled by a solenoid pressure which is controlled by varying a duty ratio of an electric current supplied to a solenoid.
The solenoid pressure is not always supplied to the lock-up control valve. Specifically, it is supplied to the lock-up control valve during operation with a gear position higher than first speed position only. A constant pressure is supplied to the lock-up control valve during operation with the first speed position or where R, N or P range is selected, thus holding the lock-up control valve in a lock-up clutch disengagement position. Such shift is effected by a shuttle shift valve. Specifically, the shuttle shift valve is switched by a second speed brake pressure which is present during operation with the second to the fourth speed positions. The shuttle shift valve allows the solenoid pressure to be supplied to the lock-up control valve when the second speed pressure is present, while it allows the constant pressure to be supplied to the lock-up control valve when the second speed pressure is not present. During operation where it is desired to engage the lock-up clutch, even if the solenoid is kept operated owing to a failure of the solenoid or a control unit, the lock-up control valve is held in the lock-up clutch disengagement position, since the shuttle shift valve ensures the supply of the constant pressure to the lock-up control valve as long as the second speed pressure is absent. In this manner, the vehicle can perform an emergency running such that the lock-up clutch is disengaged during operation with the first speed position although it is kept engaged at a speed position higher than the first speed position.
A problem encountered with the above-mentioned system is that the shuttle shift valve is needed and thus a setting space of the shuttle shift valve and that of hydraulic passages for connecting this valve with the lock-up control valve, etc., are required, resulting in enlargement of the overall dimension and increased manufacturing cost.
An object of the present invention is provide a system for controlling a lock-up clutch wherein the shuttle shift valve of the above type is eliminated.